Multi-wavelength lasers are key building blocks for communication networks that use multiple wavelengths on a single optical fiber. Integrating the transmitters, e.g. laser diodes, and the wavelength multiplexer on a single chip allows cost saving and leads to smaller components. FIG. 1 schematically shows a known integrated multi-wavelength laser 1 comprising an array of Distributed Bragg Reflector (DBR) lasers 2 and an integrated arrayed waveguide grating (AWG) or Phased Array (PHASAR) 3. Each of the lasers of the array of lasers 2 emits an optical input signal at a different wavelength and each of these lasers is in optical communication with one input waveguide of a plurality of input waveguides 7 of the AWG 3. The individual optical input signals of the lasers of the array of lasers are multiplexed into a wavelength division multiplexing (WDM) optical output signal comprising the sum of the individual input wavelengths (Σλ). The WDM optical output signal is received by a common output waveguide 8.
The most efficient multiplexers are based on an Arrayed Waveguide Grating (AWG) 3 which is extensively described in a prior art document by M. K. Smit and C. van Dam, “PHASAR-Based WDM-Devices: Principles, Design and Applications”, IEEE J. of Sel. Top. In Quant. Electr., Vol. 2, No. 2, June 1996, or an Echelle grating. Other methods of coupling multiple lasers into a single output usually have higher losses, especially for large numbers of lasers.
FIG. 2 schematically shows a basic layout of a known AWG 3. FIG. 2 shows that the AWG 3 has an input slab region or input free propagation range (FPR) 4 and an output slab region or output FPR 5 that are in optical communication by an array of waveguides 6 having a length increment from one waveguide to the next. Furthermore, the blow up in FIG. 2 schematically shows the arrangement of the waveguides of the array of waveguides 6 at the output section of the input FPR 4 of the AWG 3.
Furthermore, it can be seen from FIG. 2 that the input slab region 4 of the AWG 3 is also in optical communication with a plurality of input waveguides 7, whereas the output slab region 5 is also in optical communication with a central output waveguide 8 for receiving a WDM optical output signal. The AWG 3 further has a central channel wavelength, λ0, and a number of channels that are spaced apart by a channel spacing, Δλ.
In order to have efficient power coupling of the optical input signals from the individual lasers of the array of lasers 2 to the common output waveguide 8, the different wavelengths emitted by the lasers need to be aligned with the AWG passbands.
A known method of achieving this is to measure the light output of the device 1 and then maximize the output power by tuning the individual laser wavelengths. In the remainder of this application the expressions tuning of the laser wavelengths and tuning the lasers are used interchangeably.
Tuning the lasers can be done for example by injection of electrical current into one or more specific tuning sections of the lasers. Other tuning methods include applying a reverse bias or changing the temperature of the lasers. It is however difficult to measure the output power efficiently without interrupting the signal. Therefore, a disadvantage of the known method is unacceptable downtime of a WDM system due to maintenance or servicing. As a result, the downtime of the WDM system would have to be carefully scheduled.